This invention relates generally to the lockbolt art, and more specifically concerns lockbolt collars which are used in combination with lockbolts to securely clamp two or more overlapped parts together.
Lockbolts are commonly used where high strength fasteners are required, such as in aircraft fuel tanks and aircraft cabins. Typically, a lockbolt collar is provided for use with a lockbolt which, when forced against the typically plate-like members forming the structure (e.g., fuel tanks or aircraft cabins) over that portion of the lockbolt which extends through the overlapped members and properly swaged to mate with a lockbolt, provides a reliable, high-strength fastener for the structure. Such a combination lockbolt and collar is disclosed in U.S. Pat. No. 3,094,017 to Champoux et al. In many such applications, an interference fit between the plate-like members and the lockbolt is required in order to insure the necessary tightness of the joint between the members and the lockbolt. To provide such an interference fit, the lockbolt itself is made slightly larger in diameter than the openings in the members through which the lockbolt is to fit. The lockbolt is forced through the openings, thereby slightly deforming the material in the members adjacent the openings to accommodate the lockbolt, and creating an extremely tight (i.e., fuel tight) interference fit between the members and the lockbolt. In many applications, it is essential that such an interference fit be created and maintained for proper operation of the structure in which it is used.
It has been discovered, however, that the tightness of the interference fit between a lockbolt and the members is reduced, and in some cases eliminated, during the process of swaging the collar onto the lockbolt. In many applications, this can lead to the rejection or failure of the structure in which the interference fit is used. The present invention is directed to a solution of this reduction in the tightness of the interference fit caused by the process of swaging.
In accordance with this invention, it has been discovered that the loss of interference fit is caused by a collar material flow phenomenon which occurs during the swaging process. After the collar is fitted over the lockbolt, and the swaging die is positioned adjacent the end of the collar farthest from the overlapped members, swaging begins by forcing the swaging die against the collar toward the plate members. During swaging, some collar material is forced into the annular grooves of the exposed ribs of the lockbolt, resulting in a secured mating of the collar and the lockbolt. The remainder of the collar material displaced by the swaging die, however, is displaced initially in the general direction of the nearest of the secured members, and as swaging continues, the accumulating excess of collar material, upon reaching the nearest member, first flows radially inward toward the lockbolt, and then material adjacent the lockbolt tends to flow longitudinally of the lockbolt away from the secured members.
Typically, during the initial stages of swaging, the lockbolt is gripped by the swaging die and a sufficient longitudinal force is exerted on the lockbolt to securely clamp the two members together between the head of the lockbolt and the collar. As deformation of the material begins, the first lockbolt groove (i.e., that groove nearest the initially deformed end of the collar) is substantially filled by collar material, which has the effect of locking the lockbolt, the collar, and the overlapped parts together in that spatial position. At this point, the longitudinal tension force exerted on the lockbolt by the swaging die itself is preferably released, and deformation of the collar is continued. The longitudinal tension exerted on the lockbolt solely by the swaging process is sufficient to produce the reverse material flow phenomenon, and subsequent elongation of the lockbolt.
In accordance with the above, it is an object of the present invention to provide a lockbolt which overcomes the adverse effects of the prior art lockbolt collars described above.
More specifically, it is an object of the present invention to provide a lockbolt collar, which, when swaged to mate with a lockbolt will not cause a loss of interference fit between the lockbolt and the parts which are being clamped by the lockbolt and collar combination.
It is another object of the present invention to provide a lockbolt collar, which, when swaged to mate with a lockbolt, will not result in an undesirable elongation and attendant cross-sectional reduction of the lockbolt.
It is a further object of the present invention to provide a lockbolt collar which, when swaged to mate with a lockbolt, results in the accumulating excess collar material being left in the vicinity of the near member rather than being forced in a reverse flow back along the lockbolt.